130 THE ESSEX NATURALIST
More recently (in fact, since the last war), internal clocks
have been discovered within the objects themselves, or in
their immediate surroundings, and I shall devote the greater
part of my Address to a consideration of these recent
dramatic discoveries.
In the older methods, a simple principle, but one of great
importance for relative dating, is that if one object is found
deeper in the ground than another, then the first object was
buried earlier in time than the second. This rule can be
misleading in certain special cases, as we shall see; upon
occasion, error may be due to a careless excavator, who
dislodges the first object from the side of his trench and then
discovers it at his feet on the floor of the trench, at a lower
level than the second object, which was in fact buried earlier.
This, alas, is not so wildly improbable as to be dismissed
without consideration. Again, there are cases when two
objects are found more or less together at the same level,
yet to assume that they are of the same age may be quite
wrong. One may have originated and been buried in far
more ancient times, have been eroded by a stream from
ancient strata and swept into a new river gravel formation
alongside a piece of bone, for example, which is contem-
porary with the new gravel, both then become re-buried
together to puzzle the archaeologist twenty-thousand years
later still.
Or, indeed, cases are known of neolithic implements
being included, apparently deliberately, with burials of less
than two thousand years ago. But, in general, the rule
"the deeper, the older" is obvious and reliable.
In an address which must be read in less than an hour,
it is necessary to be selective ; therefore, I shall not touch
upon the age of things but a few hundred years old, nor
shall I refer to geological strata of vast age, except those
of the last million years during which Man or a similar
animal has lived. Many devices available to the older
physical knowledge of a century ago have been pressed into
the service of Geology. One aim of Geology is to give an
assessment of the period of time which has elapsed since
the various rock formations were laid down, and particularly
in the case of the quaternary strata which contain the